CA2169802C

CA2169802C - Integrated air separation process

Info

Publication number: CA2169802C
Application number: CA002169802A
Authority: CA
Inventors: Ravi Kumar
Original assignee: BOC Group Inc
Current assignee: Linde GmbH
Priority date: 1995-05-24
Filing date: 1996-02-19
Publication date: 1999-11-02
Anticipated expiration: 2016-02-19
Also published as: KR0179464B1; TW349030B; DE69618711T2; EP0744205A2; MY117675A; SG44736A1; CN1150917A; AU5058896A; JPH08318120A; KR960040429A; CN1074938C; DE69618711D1; US5560763A; IL117158A; AU711711B2; EP0744205B1; IL117158A0; CA2169802A1; ZA961566B; EP0744205A3

Abstract

An air separation method in which a pre-purification unit is provided with threeor more beds which are subjected to a pressure swing adsorption process. Each of the beds is subjected to feed, pressurization, purge and repressurization stages. The purge stages are conducted with a waste stream from the air separation unit and the duration of the purge stage is equal to the total time of the cycle divided by the number of beds.

Description

- 21 6q802 INlEGRA~ AIR SEPARAII~N ~S

R~ OUNn OF TE~ VI~ON

Ihe present invention relates to an illt~d air separation process in which a pre-purification unit is integrated with an air separation unit to ren~ve impurities fi~m the air prior to its separation. More particularly, the present invention relates to such a process in which the pre-purification unit co.~ c ad~~ t beds which opeIate in acco,dance S with a pressure swing adsorption cycle. Even more paIticularly, the present invention relates to such a method in which the pressure swing adsorption cycle in~hl~les contin~ cly s~;ecti~ the beds to a purge with a waste slreaîn such as waste nitrogerL

Air is se~u~ed into its coll~l~ents by a variety of low t~ re rectification 10 ~. In accol~ce with such processes, air is c~ essed, cooled to a t~l4~.~Llre suitable for its re~tifi~tion~ and then is in~duced into a air separation unit which c~ one or more columns in which the air is distilled into its coll~~ parts. Ihe air separation unit can contain one or more tlictill~tion columns having ~ays or pac~ing to provide i- 1~ e contact b~ lwe~n vapor and liquid phases of the air to be se~udl~
In ~ih~ to oxygen, nit~ogen and argon, the air also C~ ;1L~ higher boiling cr~ such as carbon dioxide, hy~s and water. If these higher boiling co~ are not rernoved fiom the airprior to the cooling of ~e air, such c~ lts will cc~ .oe during the cQoling of the air and form ioe in the main heat ~ch~tl~r.
20 .~lihi~lly, ~ ill~i~ CQ~ tend to c4 r~11-~ such higher boiling ~ll~n~lts as liquid column b~~ llLC, In case of hy~oc~ s, this can present an op rational ha~l In order to prevent such ~ob'~n~, pre~p~ifi~ti-~n units are provided that are integrated with the air separation plant in order to remove the higher boiling colly~l~ such as moisture, carbon dioxide and hyd~s. Such pre purification is effe~ te~ by beds 25 of adsorbent ~,~ling in accoYdanoe with pressure swing adso~ption cycles.

21 6q~02 A common plesaul~, swing adsorption cycle has feed, de~lesau,;zation, purge and re-p~esau,;~lion stages. During the feed stage, co",plessed air is fed to a bed to produce a purified air stream which is then cooled in the main heat e~chA~el. After the feed stage is co,l,~lete, the bed is deplesa~;~t to ~tmosph~ic ple..~we. The depiesa~ ;on S initi~tes desorption of the adsc"bcd i~lpu~ilies from the adsoll~nt. Depresju,;zation is completed by yu~ging the bed with a waste stream, commonly waste nitrogen when available from a lower pies~ule column of a double column air separation unit.
Thereall~ r, part of purified air from another bed undergoing a feed stage is routed to the bed that has just been subjected to a purge stage to lep,. ssul;ze the bed.
It is possible to use two beds which are operated out of phase such that while one bed is being fed, the other bed is rege.lllated and is therefore being subjected to dcplesaul;~alion, purge and rei)resaul~alion stages. Even when more than two beds are used, the beds are only purged intermittently. As will be ~1iccucce~ the inventor herein 15 has found that operational efficiencies can be realized by opelalillg a pre~ ,e swing adsorption cycle so that a waste stream is continuously being used to purge a bed undergoing the purge stage of the cycle.

SUMMI~Y OF THF INVF~TION
The present invention provides an integrated air separation method. In accordance with this metho~1, air is s~aled by co~ )lessing a feed air stream, cooling the feed air stream to a hlll~l~tu~ suitable for its rectification and then r~cliryillg the air so that a waste stream is produced. The waste stream is warmed prior to its fur~er utilization.
25 The feed air stream is pre-purified prior to the cooling thereof by plesaule swing adsol~Jtion process. The ~ ,aulc; swing adsoll~tion process compri~s subje~,ling each of at least t_ree adsorbent beds configured to adsorb illl~ul;lies to a cycle including a feed stage to adsorb the i,ll~;lies at presau,e. Then ~,l, the beds are ~genel~ted by a deplesau,;zation stage to initiate desorption of the i",~u,;lies, a purge stage to complete 30 deso,~tion of the h~lyu~;lies and a ,eple~ At ;on stage to reprc;,aul;ze each of the at least three beds back to the pl~,sa~.,. The purge stage is con~uctç~ by introducing the waste 21 69~02 str~m into each of ~e ~ree ad~ bais for ~e d~ation of ~}e time interval equal to~e total time i~al of ~e cycle as applied to eacll of d e least ~ree beds divided by a total n~ of d}e adso~bent beds. As a result, ~e p~ge stream is a~inluylcly being used to subject a bed to a purge stage.

In a two bed syskm, since one bed is proch~ while ano~ bed is being leg~ed, ~e exists a time period in which, althou~h available, the purge strearn is not being used to purge a bed Hence, the present invention requires three Of mo~e beds so that the purge stream is Crnl1;"~ ly being used The effect ofthe present invention 10 is to i~ ea3e the ~Ulllt oftime that each bed is ~*~ed to the purge stream so that the c ~n-~lneion of the p~ge stream, more i~ulriti~ have be~ des~l~ fiom the bed than by prior art mf~thl~c As a result, more impurities can be adsd.l~d in the _rst ;~ n~e The application of the present invention is l~ f~n~ to re~ce the ~nKulnt of aLcorbent required for a pre purific~ti~n unit of an air s~u~ion plant.
RRTF.F nF~C~TPrION OF T~F T)RAWr~('T

While the specification ccn~ es with claims lictin~tly pointing out the subjectma~ter that Applicant regards as the invention, it is believed that the invention will be 20 better lmll~tQod when taken in co ~ on with the A- ~n~nying hawi~l~ in which:

Fi~ 1 is a S~'~h nU~;c of an air s~ion plant for car~ying out a n~th~ in acco~ ce with the present invention;

Fi~ 2 is a ~ ;c of a prep~ - unit in acconda~ with the present inve~ion; and Fig. 3 is a sr~ 1 ;G cycle diagrarn illùs~ing a ~ swing adsol~lion process in accoi~ with the present inventio~L

2~69~02 r)FTA~ F;n nFA~ON

W~ ,f~K:e to Fi~ 1 an air s~ion plant 1 is illu~11~ed for s~lg air.
p air is Slk~red in a filt~ 10 to remove dust ~licks and ~e like and is dlen S con~ in a COu SS~ 12. Ihe heat of coll~essioll ~c~ced d~u~h ~e co.l4~i~n of the air is removed by an ~ n DlEr 14. Ilb~, the air is purified in a p~p~ific~ti~ unit 16 and is ~ cooled in a main heat eYrh~ 18 to a t~
sl-h~'e for its rectifi~tir~n within air s~ion unit 20.

Air s~d1ion unit 20 can consist of a single ~lictill~ti~n column that is used tos~e ~e air into an oxyg~rich liquid column l~~ s and a nitrogen-rich or even a highpurity tower o~ll~ Ihe present invention is not limited by the actual ~i~till~tion process used or the number of rlictill~tic~n columns used in c~rying out the process. For in~t~P~, air s~al~ion unit 20 could be a double column in which higher and lower15 pressure columns are operatively ~scri~te~ with one another by a co.-~-reboiler.
Ihe coln~ssed and cooled airwouldbe introduced into the bottom ofthe higherpressure column to produce a nitrogen rich tower ove~head and an oxygen-rich column bottoms, known in the art as crude liquid oxygen. Ihe crude liquid oxygen would be further refined in the lower pressure column into a ~ nitrogen tower overhead and a liquid 20 oxygen column ~llæ. Air s~ion unit 20 coalld also be a single column o~ygen or ni~Dgen geI~.

Fo~purposes of c~ n, it will be ~lrn~d ~at air separation unit 20 produces a waste nhnDgen sire~un 22. HiDwever, if an oxygen were not desired, waste stream 22 25 could be formed from ~n o~ygen directly above the sump of a do~le column air s~ion ur~ ~iti~lly, air ~ )n unit 20 produces a pr~uct s~n 24 which in a 1l~.~ well known in the art could be, liquid o~ygen to be ~d within rnain heat c ~ 18 or ~1~ oxygen or ni~og~l to be ~ d within rnain heat ~ 18 to an~ient con~

2 1 69~02 W~ ~e to Fig. 2 pre plllifi~ti~ ~it 16 is p~vided wi~ firs~ second and ~ird beds 26, 28 and 30. E~ of ~e b~ds C~ ;.L~ an ads~l~ to adsofb ~e higher boiling im~ities, for ~ e, activated ~ (AA) 7 x 12 nxs}l beads ~ fi;ul~lredby La P~xhe C~mica~ P.O. Box 1031, ~r~ne HGghway, Ba~on F~oug~ I~L 70821-1031.
S (~jll4nw~ed a~ from ~[1L~OOI~ 14 enk~s a h~ 32 and is ~ n~i~lly ~ed each of f~s~ seoond and ~rd a~ beds 26-30 to p~oduce a F~odh~ sbx~rn wh~ch is d~ ed fiom a F~odhK~ header 36 to n~un heat ~ n~ 18.

F~rh of ~e ~ s~ and ~ird beds 26-30 is subjected to feed, 10 dl~i~tion, purge and then ~n~i~on stages by o~ation of on/offvalves 38-66, whic~ ~h~l~h not illus~ated could be l~ ly activated by a prog~ ~le or analog oDnb oller. W~ ~itic)n~ f~lce to Fig. 3, a valve ~~ e chart follows to d~;be the ope~tion of valves 38~6 to effect pressure swing adsorption process illus~ated Fig 3. Op~ valves are ~ign~t~ by ~e sy~bol "O".

VALVE SEQUE~NOE CHART
Valve 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 Nurnber r~lTIil~
0-0.5 0 0 0 0 0 0.5-12 0 0 0 0 0 0 12-12.5 0 0 0 0 0 12.5-24 0 0 0 0 0 0 2424.5 0 0 0 0 0 24.5-36 0 0 0 0 0 0 2 1 ~9802 ~ ~e f~ half min~e, valves 38, 46, 48, 52 and 62 ~L As a r~ult, ~e feed air s~eam, formed from col.4n~ed air cooled in aP.ercooler 14, feeds to f~;t a~l~l~lt bed 26 to subject first ~~ bed 26 to a feed stage. Since valve 62 is also ~ the p~ct is ~i;scl~ed to pr~duct head~ 36 from f~rst a~ bed 26. Valves 46 and 5 52 are open to subject the sec~d ~hnl~ b~d 28 to the purge stage. Open valve 48 allows ~ird ~a 30 to be subjected to ~e d~e~ni7~ n stage.

At dle sl~ time interval, l~tw~l a half and 12 mimlt~, f~rst a~hl~l~ bed 26 c~)ntimus to be SUI~J~t~ to the fe~d stage. Second ads~oent bed 28 is now, however, 10 being sul;s~e~ to a l~rization stage. To this end, valves 56 and 58 are open to subject second a~ l~lt bed 28 to part of the product stream at pressure. Valves 54 and 48 have ~ed to subject third adsorbent bed 30 to the purge stage.

D~ing the time interval from 12 to 12.5 minlltes f~rst adsorbent bed 26 is now 15 being subjected to d~lrizatioiL To this end, vatve bed 44 is ~L Second adso~ t bed 28 is now subjected to the feed stage by setting valves 40 and 64 in the open positiC)n Third ad~l~.lt bed 30 c- ntinlles to be subjected to purge.

In the next time interval from 12.5 to 24 minllt~s f~t adsor~ent bed 26 is 20 s~J~cted to the purge stage by ~.;~g valves S0 and 44. T~lird adsorbent bed 30 is ~nized by setting valves 58 and 60 in the ~pen position.

The next s~ g time interval, from 24 to 24.5 minl~ has second adsoll~lt bed 28 being S1~Je~t~d to a d~ ri7~i~n stage by setting valve 46 in the ~pen 25 ~;~ At the same time, dlird ads~l~lt bed 30, whic~ previously was l~llri is now subjected to the feed stage by setting valves 42 and 66 in the open position.

The last time interval, from 24.5 to 36 minllt~, has f~rst adsoll~l bed 26 beingi7fYl (Ixior to being brought on line) with pa~t of the pro~ct st~eam being 0 pr~ced fiom third adsull~l~ bed 30. Valves 56 and 60 are set in the ~ position for 2 1 6'i~02 this pulpose. Second a~sorbent bed 28 is now subjected to the purge stage by ~pening valve~s 52 and 46.

The above-l~f~ ce se~ cl ntin~ lsly r~s with beds acting in an S adsorbing capacity followed by reg~ ~ion. As can be appreciated, in the illustrated errbo~lim~t the time that each bed is e~osed to the feed stage is equal to the purge stage and the total time of the cycle is equal to about three times the time of the purge stage or for that rnatter the time of the feed stage.

The following is a calculated exarnple to show the adsorbent savings when con~ ing a pressure swing adsorption cycle for a pre-purification unit in accor~lce with the present invention as co,l~&~d with prior art two bed mPth~. It can be seen fiom this c ~l~lc that con~d to a two bed method, roughly 4,808 Kg of adso are saved with the three bed m~th~d of the present invention.
~ODUCT: CO2= l ppm ped~
AMORBENT: Act~vded AI~una (AA) 7xl2 mesh beads CASE # - # OF BEDS PRIOR ART 2 SUGGESTED 3 Weight of Adsorbent 28,852 24,044 (Kg.) Weight of Adsorbent/Bed 14,426 8,015 Kg.) Inner Did~ of Bed, 2.7 2.25 Me~rs Lengdl of Bed, Meteas 3 2.4 % Purge/Feed 41.5 50 % Vent Loss 1 0.5 %Ma~L ~surization/ 9 1.3 Feed Flow Feed Time, milL 12 12 Time D~ul;~i~, 0.5 0 5 mirL
rlme P~ge,mirL 10 12 Time R~pressurization, l.S 11.5 min.
Total Cycle TimPmin- 24 36 For p~poses of ~is e~rnple, the feed pressure and te,~ re is about 3 ~tn~ .cs at about 35 C. The feed rate is about 140 ~ cubic meters per minute.
The purge stream has a pressure of about 1 bara and a tc~ re of about 35 C. The15 flow rate for the purge strearn is about 71 ~d cubic meters minute.

In addition to dle ar~ ioned advantage of lower adsorbent requi,~ cllt, maximurn fl~ti~n in pro~ct flow rate from pre-purification unit 16 is reduced from about 9 to 1.31. This improves the operation of the dov~ u~l air separation unit 20.
While ~e present invention has been des~ibe~l with lef~ ",ce to a pl~,f~
~nho-1irnf~t as will oca~r to those skilled in the art, numerous r)l~n~c, ~ltiition~c and omissions can be made without d~,~ling fiom the spirit and scope of the present inventiorL

Claims

1. An integrated air separation method comprising;

separating air by comprising a feed air stream, cooling the feed air stream to atemperature suitable for its rectification, rectifying the air so a waste stream is produced, and warming said waste stream; and prepurifying the feed air stream prior to said cooling thereof by a pressure swing adsorption process to produce a purified feed air stream;

the pressure swing adsorption process comprising subjecting each of at least three adsorbent beds to a cycle including a feed stage to adsorb impurities contained within the feed air stream and thereby to produce said purified feed air stream, a depressurization stage to initiate desorption of said impurities, a purge stage to complete desorption of said impurities and a repressurization stage to repressurize each of said at least three beds;

said purge stage being conducted by introducing said waste stream into each of said at least three adsorbent beds for a duration of a time interval equal to a total time interval of said cycle as applied to each of said at least three adsorbent beds divided by a total number of said adsorbent beds, whereby said purge stream is continuously being used to subject said adsorbent beds to said purge stage.

2. The integrated air separation method of claim 1, wherein said waste streamcomprises a waste nitrogen stream.

3. The integrated air separation method of claim 1, wherein said depressurization stage is conducted by depressurizing each of said at least three beds to atmospheric pressure.

4. The integrated air separation method of claim 1, wherein said repressurization stage is conducted by introducing a part of said purified feed air stream into each of said at least three beds.

5. The integrated air separation method of claim 1, wherein said total number of beds is equal to three.

6. The integrated air separation method of claim 5, wherein said total time interval is equal to about three times a feed time interval of said feed stage.

7. The integrated air separation method of claim 6, wherein said time interval of said purge stage is about equal to said feed time interval.

8. The integrated air separation method of claim 7, wherein said waste stream comprises a waste nitrogen stream.

9. The integrated air separation method of claim 7, wherein said depressurization stage is conducted by depressurizing to atmospheric pressure.

10. The integrated air separation method of claim 7, wherein said repressurization stage is conducted by introducing a part of said purified feed stream into each of said at least three beds.